Reorg (#386)

* Step 1: move electorn into `app/`.

This is to comply with the suggested directory format of
`electron-builder`: https://github.com/electron-userland/electron-builder#two-packagejson-structure

* Step 2: add build directory with icon files for mac / windows

* Step 3: move all development (web) assets into main directory

* Step 4: add `build` namespace to dev `package.json`

* Step 5: move all dev dependencies into dev file and get rid of
old electron packagers in favor of `eletorn-builder`

* Step 6: target build inside `app/` as everything else is excluded at build time

* Step 7: remove old stuff!

* Step 8: update README

* turn off asar for `child_pty`